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From: Ross Lagerwall <ross.lagerwall@citrix.com>
To: Juergen Gross <jgross@suse.com>, <xen-devel@lists.xenproject.org>,
	<linux-kernel@vger.kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: [PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices
Date: Wed, 17 Feb 2021 13:29:19 +0000	[thread overview]
Message-ID: <6818fcde-abab-1250-119c-d0ccb8c80488@citrix.com> (raw)
In-Reply-To: <20210211101616.13788-9-jgross@suse.com>

On 2021-02-11 10:16, Juergen Gross wrote:
> For avoiding read- and write-tearing by the compiler use READ_ONCE()
> and WRITE_ONCE() for accessing the ring indices in evtchn.c.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> V2:
> - modify all accesses (Julien Grall)
> ---
>  drivers/xen/evtchn.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
> index 421382c73d88..620008f89dbe 100644
> --- a/drivers/xen/evtchn.c
> +++ b/drivers/xen/evtchn.c
> @@ -162,6 +162,7 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
>  {
>  	struct user_evtchn *evtchn = data;
>  	struct per_user_data *u = evtchn->user;
> +	unsigned int prod, cons;
>  
>  	WARN(!evtchn->enabled,
>  	     "Interrupt for port %u, but apparently not enabled; per-user %p\n",
> @@ -171,10 +172,14 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
>  
>  	spin_lock(&u->ring_prod_lock);
>  
> -	if ((u->ring_prod - u->ring_cons) < u->ring_size) {
> -		*evtchn_ring_entry(u, u->ring_prod) = evtchn->port;
> +	prod = READ_ONCE(u->ring_prod);
> +	cons = READ_ONCE(u->ring_cons);
> +
> +	if ((prod - cons) < u->ring_size) {
> +		*evtchn_ring_entry(u, prod) = evtchn->port;
>  		smp_wmb(); /* Ensure ring contents visible */
> -		if (u->ring_cons == u->ring_prod++) {
> +		if (cons == prod++) {
> +			WRITE_ONCE(u->ring_prod, prod);
>  			wake_up_interruptible(&u->evtchn_wait);
>  			kill_fasync(&u->evtchn_async_queue,
>  				    SIGIO, POLL_IN);

This doesn't work correctly since now u->ring_prod is only updated if cons == prod++.

Ross


  reply	other threads:[~2021-02-17 13:29 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-11 10:16 [PATCH v2 0/8] xen/events: bug fixes and some diagnostic aids Juergen Gross
2021-02-11 10:16 ` [PATCH v2 1/8] xen/events: reset affinity of 2-level event when tearing it down Juergen Gross
2021-02-14 21:17   ` Julien Grall
2021-02-11 10:16 ` [PATCH v2 2/8] xen/events: don't unmask an event channel when an eoi is pending Juergen Gross
2021-02-11 10:16 ` [PATCH v2 3/8] xen/events: avoid handling the same event on two cpus at the same time Juergen Gross
2021-02-14 21:34   ` Julien Grall
2021-02-15  6:55     ` Jürgen Groß
2021-02-15 21:35   ` Boris Ostrovsky
2021-02-18 11:47     ` Jürgen Groß
2021-02-11 10:16 ` [PATCH v2 4/8] xen/netback: fix spurious event detection for common event case Juergen Gross
2021-02-11 10:53   ` Jan Beulich
2021-02-11 14:15   ` Paul Durrant
2021-02-11 15:24   ` Wei Liu
2021-02-11 10:16 ` [PATCH v2 5/8] xen/events: link interdomain events to associated xenbus device Juergen Gross
2021-02-11 14:16   ` Paul Durrant
2021-02-11 10:16 ` [PATCH v2 6/8] xen/events: add per-xenbus device event statistics and settings Juergen Gross
2021-02-15 21:54   ` Boris Ostrovsky
2021-02-11 10:16 ` [PATCH v2 7/8] xen/evtch: use smp barriers for user event ring Juergen Gross
2021-02-11 10:16 ` [PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices Juergen Gross
2021-02-17 13:29   ` Ross Lagerwall [this message]
2021-02-18 11:46     ` Jürgen Groß

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